The invention relates to a desulfurization process and a catalytic isomerization process for hydrocarbons, particularly paraffin-containing hydrocarbon feedstreams.
Natural or straight run gasoline, i.e., naphthas, contain normal paraffins (n-paraffins) which have relatively low octane numbers. In the catalytic isomerization of paraffins, such as n-hexane and n-pentane, to equilibrium mixtures of branched chain isomers, the octane rating of the paraffins is substantially increased. Previous processes directed to paraffin isomerization have employed acid- promoted aluminum chloride catalysts, Y-type crystalline zeolite catalysts containing an elemental metal of Group VIII of the Periodic Table (U.S. Pat. Nos. 3,236,761 and 3,236,762), synthetic mordenite catalysts containing highly-dispersed platinum or palladium in the presence of hydrogen (U.S. Pat. Nos. 3,527,835 and 3,299,153), stabilized Y-sieve hydrogen zeolite compositions (U.S. Pat. No. 3,354,077), and the like. Commonly used commercial isomerization catalysts contain significant proportions of relatively expensive platinum or palladium metal components.
The hydrocarbon feedstream to an isomerization reactor is generally treated (hydrotreated) to remove sulfur and nitrogen compounds that rapidly deactivate (even poison) the relatively expensive isomerization catalyst. A common method of treating the feedstream to reduce the sulfur content is by hydrodesulfurization wherein the feed is contacted a sulfur-tolerant hydrogenation catalyst in the presence with of hydrogen. Although good sulfur removal may be achieved with hydrodesulfurization units operating under severe conditions, the efficiency of such units is ultimately limited by equilibrium and/or kinetic considerations and further desulfurization is often necessary in downstream processes.
Downstream processes employing relatively inexpensive, catalytically active materials further reduce the sulfur content of a hydrocarbon by "absorbing" sulfur therefrom under either hydrogenative or nonhydrogenative conditions. The absorbent material may contain a metal component, such as nickel, copper, or silver, and the feedstocks generally treated are reformer feedstocks, particularly naphthas. Typical of such processes is that disclosed in U.S. Pat. No. 2,755,226 to Annable wherein a bed of copper molybdate pellets is used to reduce the sulfur content of naphthas. Similarly, in U.S. Pat. No. 4,224,191 to Bishop III, U.S. Pat. No. 4,204,947 to Jacobson et al. and U.S. Pat. No. 4,582,819 to Miller, the use of copper components supported on conventional carriers is disclosed for reducing the sulfur content of reformer feedstreams. Also, processes disclosed in U.S. Pat. No. 4,419,224 to Miller include reducing the sulfur content of reformer feedstreams with promoted nickel components on alumina carriers.